/*
*	ID : pallab81
* 	PROG : 11404
* 	LANG : C++
* 	2012-06-18-12.23.07 Monday
*
* 	"I have not failed, I have just found 10000 ways that won't work.
*/

#include <iostream>
#include <algorithm>
#include <vector>
#include <sstream>
#include <fstream>
#include <string>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <functional>
#include <bitset>
#include <iomanip>

#include <ctime>
#include <cassert>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <climits>
#include <cstring>
#include <cstdlib>

using namespace std;


#define foR(i1,st,ed) for(int i1 = st , j1 = ed ; i1 < j1 ; ++i1 )
#define fo(i1,ed) foR( i1 , 0 , ed )
#define foE(i1,st,ed) foR( i1, st, ed+1 )
#define foit(i, x) for (typeof((x).begin()) i = (x).begin(); i != (x).end(); i++)
#define bip system("pause")
#define Int long long
#define pb push_back
#define SZ(X) (int)(X).size()
#define LN(X) (int)(X).length()
#define mk make_pair
#define SET( ARRAY , VALUE ) memset( ARRAY , VALUE , sizeof(ARRAY) )
#define line puts("")

template<class T1>
inline void debug(T1 _x) {
    cout<<_x<<'\n';
}
template<class T1, class T2>
inline void debug(T1 _x,T2 _y) {
    cout<<_x<<' '<<_y<<'\n';
}
template<class T1, class T2, class T3>
inline void debug(T1 _x,T2 _y,T3 _z) {
    cout<<_x<<' '<<_y<<' '<<_z<<'\n';
}
template<class T1, class T2, class T3, class T4>
inline void debug(T1 _x,T2 _y,T3 _z,T4 _zz) {
    cout<<_x<<' '<<_y<<' '<<_z<<' '<<_zz<<'\n';
}
template< class T1>
inline void debug(T1 _array,int _size) {
    cout<<"[";
    for (int i=0; i<_size; ++i) {
        cout<<' '<<_array[i];
    }
    puts(" ]");
}
inline bool CI(int &_x) {
    return scanf("%d",&_x)==1;
}
inline bool CI(int &_x, int &_y) {
    return CI(_x)&&CI(_y) ;
}
inline bool CI(int &_x, int &_y, int &_z) {
    return CI(_x)&&CI(_y)&&CI(_z) ;
}
inline bool CI(int &_x, int &_y, int &_z, int &_zz) {
    return CI(_x)&&CI(_y)&&CI(_z)&&CI(_zz) ;
}
inline void wait( double seconds ) {
    double endtime = clock()+( seconds* CLOCKS_PER_SEC );
    while( clock() < endtime ) {
        ;
    }
}
int dp[1001][1001];
string str;
int ln;

int go(int l, int r) {
    if(l>r)return 0;
    int &b= dp[l][r];
    if(b!=-1)return b;

    b=0;

    if(str[l]==str[r]) {
        b=max(b, (l==r?1:2)+go(l+1,r-1));
    } else {
        b=max(b, go(l+1,r));
        b=max(b, go(l,r-1));
    }

    return b;
}
void bk(int l, int r, int ret) {
    if(l>r)return ;

    for(char ch='a';ch<='z';++ch){
        for(int i=l;i<=r;++i){
            if(str[i]==ch){
                for(int j=r;j>=i;--j){
                    if(str[j]==ch){
                        if( i==j ){
                            if(go(i,j)==ret){
                                cout<<ch;
                                bk(i+1,j-1,ret-1);
                                return ;
                            }
                        }
                        else{
                            if(go(i,j)==ret){
                                cout<<ch;
                                bk(i+1,j-1,ret-2);
                                cout<<ch;
                                return ;
                            }
                        }
                    }
                }
            }
        }
    }
}

inline void Solve() {

    ln = LN(str);
    SET(dp,-1);
    int v=go(0,ln-1);
    //debug(v);
    bk(0,ln-1,v);
    line;
}
#define FILE
int main() {
#ifdef FILE
    freopen("/home/xerxes/Documents/11404_2/in","rt",stdin);
#endif

    while( cin>>str ) {
        Solve();
    }

    return 0;
}
